The objectives of this project are: To determine the molecular basis of the change in glycoprotein surface protein (antigenic variation) in Trypanosoma congolense. We will look both at the genotypic and phenotypic correlates of antigenic variation. Two amphotropic murine leukemia virus variants have been isolated; one associated with tumors of early B cells (lymphoreticular tumors) and another associated with late B cell tumors (myelomas). We will look at the surface glycoproteins and other viral proteins to determine if tropism is correlated with structural changes.